MMFSK-EOD System Using Binary Hopping Pattern in FH/SS Communications

Transcription

1 JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 2, MARCH MMFSK-EOD System Using Binary Hopping Pattern in FH/SS Communications Suguru Hasegawa Nagaoka University of Technology, Department of Electrical Engineering, Kamitomioka-machi, Nagaoka-shi, Niigata, , Japan Tel/Fax: /9500, Shin ichi Tachikawa and Gen Marubayashi Nagaoka University of Technology, Department of Electrical Engineering, Kamitomioka-machi, Nagaoka-shi, Niigata, , Japan Tel/Fax: /9500, Prof.Emeritus, Nagaoka University of Technology, Japan 1-104, Suzuki-cho, Kodaira-shi, Tokyo, , Japan Tel/Fax: , Abstract In this paper a detailed explanation of our newly proposed frequency hopped spread spectrum system named MMFSK-EOD System Using Binary Hopping Pattern (8) is presented. First, to make clear the concept of FH/M-ary Multilevel FSK modulation (abb. FH/ MMFSK or MMFSK), the waveform and the spectrum differences between MFSK and FH/MFSK or FH/MMFSK are reviewed. Next, to understand the error propagation phenomena from the first demodulation stage to the second demodulation stage, system principle of FH/MMFSK using EOD is explained of some length. In an EOD system, a hopping pattern is synthesized from a group A hopping pattern and a group B hopping pattern, which are different properties on even and odd levels. Finally, to prevent the error propagation novel binary hopping pattern method is proposed, and its effectiveness on improving error performance is shown by illustrative computer simulations. Some additional improving method in the design of group A hopping pattern are also shown. Index Terms MMFSK, hopping pattern, Even-Odd discrimination method (EOD), error performance I. INTRODUCTION Spread spectrum system is an attractive communication system for transmission through inferior medium such as mobile communications and powerline communications. However its poor spectral efficiency prevents wide applications. When its spectral efficiency is considerably improved wide application field will be envisaged. Based on the idea, we have been studied to improve the spectral efficiency of the frequency hopping spread spectrum system by using plural number of hopping patterns instead of single hopping pattern in existing frequency hopping system. We call the novel system as MMFSK (M-ary Multilevel FSK). In MMFSK we transmit Log 2 mh bits every T second instead of log 2 m bits in ordinary MFSK, where m is the number of frequencies used and h is the number of hopping patterns used. For simplicity we use the word level instead of frequency as later explained in the paper. In MMFSK system the more increase the number of hopping patterns the more the transmission rate can increase, however, the circuit complexity increases and system realizability becomes a problem. To avoid circuit complexity increase, EOD (Even Odd Discrimination method) hopping pattern synthesis method was proposed. In an EOD system, a hopping pattern is synthesized from a group A hopping pattern and a group B hopping pattern, where group A hopping pattern includes both even number levels and odd number levels as EOEEO and group B hopping patterns is composed of even number levels only as EEEEE. When there are m 1 hopping patterns in group A and m 2 hopping patterns in group B, then, m 1 m 2 hopping patterns can be produced by m 1 +m 2 hopping patterns and the circuit can be made greatly simplified. In the EOD demodulation process, first the transmitted A group hopping pattern is discriminated by its EO pattern and subtract it from the input level signal and in the second demodulation stage remaining B group component and the transmitted level are determined through the majority logic. However, in an ordinary EOD system, there was a defect that errors arisen in the first demodulation stage directly affect the second demodulation process and deteriorate the error probability of the system. In this paper to separate errors from the first demodulation stage and the second demodulation stage, new category of group A hopping pattern named binary hopping pattern is proposed. By using the binary hopping pattern, we can remove the effect of first stage error on the second stage demodulation performance and achieve good error performance.

2 120 JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 2, MARCH 2009 II. WAVEFORMS AND SPECTRA OF MFSK, FH/MFSK, FH/MMFSK MFSK, FH/MFSK, FH/MMFSK are variations of frequency shift keying modulation schemes. In these the waveform and the spectrum of FH/MFSK and FH/MMFSK are quite the same, so that in this section the difference between MFSK and FH/MFSK or FH/MMFSK is reviewed. First, MFSK (Multiple Frequency Shift Keying) is a modulation scheme, in which, every T second one of m frequencies is selected and transmitted. The waveform is as shown in Fig.1(a). T f j f jl (b) (a) MFSK FH/MFSK, FH/MMFSK Fig.1 Waveform. As shown in Fig.1(a) frequencies are keyed every T second from f j to f i to f k and so on. There are m frequencies f 0, f 1,, f m-1 available for each f i, f j, f k and we can transmit log 2 m bits for every T second. Each keyed duration T frequency segment (tone) shows dumped sinusoidal spectrum as shown in Fig.2. (a): Wave form (b): Spectrum Fig.2 Tone and its spectrum. f i f i = f 0 +i f (i:0, 1,,m-1, f =1/T) f i1 T f i2 f ih f il As shown in Fig.1(a) f i s are equally spaced as f i = f 0 +i f (i:0,1,2,,m-1) and it is well known that the minimum spacing which satisfies the orthogonally is f = 1/T. Fig.3(a) shows the frequency arrangement of MFSK. f k1 T/L f ih = f 0 +i h L f (i h :0, 1,,m-1, f =1/T) f k t t 1/T f 0 f 1 f i f m-1 L/T W (a) MFSK f 0 f 1 f i f m-1 f (b) LW FH/MFSK, FH/MMFSK Fig.3 Spectrum. In these figures only main lobes are shown for simplicity. Necessary total transmission bandwidth of MFSK which use m tones of duration T is W = (m+1)/t. Next, consider FH/MFSK and FH/MMFSK. Both FH/MFSK and FH/MMFSK are frequency hopped spread spectrum systems which use frequency hopping pattern as a bandwidth expansion tool. Fig.1(b) shows the waveform. As shown in the figure every signal duration T is subdivided into L chips and frequencies are keyed every T/L second. Similar to MFSK a total of m frequencies are used but their frequency spacing is L times larger than that in MFSK as f i f il f 0 (i:0, 1, 2,, m-1; f =1/T) (1) Fig.3(b) shows the frequency arrangement of FH/MFSK and FH/MMFSK. Necessary total transmission bandwidth is W= (m+1)l/t as shown in the figure. Information transmission rate of FH/MFSK is the same as MFSK, that is, R=(log 2 m)/t [bits/sec]. In this system shorter keying duration does not contribute to increase the information transmission rate, but instead, bandwidth expansion makes the system robust to various disturbances and/or variations of the line as is the characteristic feature common to all spread spectrum systems. Spectral efficiency of FH/MFSK is given as R log 2 m (2) W ( m 1) L This is 1/L smaller than that of MFSK. To improve the spectral efficiency of the system FH/MMFSK was proposed. In a FH/MFSK system a fixed frequency hopping pattern is used as a bandwidth expansion tool, instead, in a FH/MMFSK system plural number of hopping patterns is used to increase the transmission rate. This M-aryzation does not affect the transmission bandwidth nor transmission power at all and the spectrum can be given by the same figure as shown in Fig.3(b). Detailed explanation of the system given in later sections. f

3 JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 2, MARCH III.SYSTEM PRINCIPLE OF FH/MMFSK USING EOD (1) M-aryzation by using plural hopping patterns In existing frequency hopping systems including FH/MFSK, frequency hopping pattern is used as a key to open the embedded information and single hopping pattern has been used, however in our newly proposed FH/MMFSK many hopping patterns are used to increase the information transmission rate. Hopping patterns are used not only as a key to open the embedded information but also as an information carrier. When we use h hopping patterns we can transmit log 2 h [bits] per every T second by hopping pattern itself. Then the spectral efficiency of FH/MFSK expressed in equation (2) changes to the following equation (3) for FH/MMFSK. R W log 2 mh ( m 1) L (3) The more increase the number of hopping patterns we can transmit the more information, however, circuit complexity may increase and the system realizability becomes the problem. To avoid the problem EOD hopping pattern synthesis method is invented. (2) Level and vector notation of the hopping pattern To simplify the discussion hereafter we will use the word level to designate the frequency. Level is the same as number i in equation (1). As shown in Fig.1(b), in FH/MFSK or FH/MMFSK systems frequency changes every T/L second in accordance with the hopping pattern change. Frequency change with time of a hopping pattern is well expressed by the level- chip matrix, ordinate of which indicates levels 0, 1, 2,, m-1 and abscissa indicates time chips 1, 2,, L. A hopping pattern in level-time matrix can also be written as (a i1, a i2,, a il ), where a ix indicates the level of chip x. A set (a i1, a i2,, a il ) can be expressed by L dimensional vector A i, the components of which are a i1, a i2,, a il. (3) EOD (Even Odd Discrimination Method) hopping pattern synthesis In an EOD system a hopping pattern is synthesized from two hopping patterns each belongs to different hopping pattern groups A or B. When there are m 1 hopping patterns in group A and m 2 hopping patterns in group B, then a total of m 1 m 2 hopping patterns can be produced from m 1 +m 2 hopping patterns. To realize the system some discrimination method of distinguishing group A and group B is necessary. In our proposed EOD system, to give distinctive features to group A and B, group A hopping patterns comprises both even number levels and odd number levels such as EOOEO, while group B hopping patterns are composed of even number levels only such as EEEEE, where E represents an even number level and O represents an odd number level. (4) MMFSK transmitter using EOD Fig.4 shows a block diagram of MMFSK transmitter using EOD. Input K bits dater stream is first divided into K 1 bits and K 2 bits. K 1 bits select a level L h out of m=2 k1 levels. A level L h is repeated over L time chips as shown in Fig.5. K 2 bits are further divided into K 21 bits and K 22 bits. K 21 bits select a hopping pattern A i out of m 1 hopping pattern generators in group A as shown in the figure. Similarly K 22 bits select a hopping pattern BBj out of m 2 hopping pattern generators in group B as shown in the figure. These selected level and hopping patterns are added chip by chip by modulo m base and finally every level is converted to tone and FSK like signal is transmitted. An example of the modulation process of the transmitter is shown in Fig.5. (5) MMFSK receiver using EOD Fig.6 shows a corresponding receiver block diagram. Input frequencies are first converted to levels and the transmitted level signal L h +A i +BBj is reproduced. In the first stage of demodulation, input level signal is subtracted by A group hopping patterns in parallel. Output of A k subtracted branch is L h +(A i -A k ) +B jb and unless A i =A k the output contains both even number level E and odd number level O, thus we can determine transmitted A i by selecting a branch which exhibit all E or all O. When A i is correctly subtracted, the input signal for the second stage becomes L h +BBj and this input signal is subtracted by B group hopping patterns in parallel and only the output of B jb subtracted branch shows a horizontal straight line L h in the receiving matrix, thus we can determine both BBj and L h. Fig.4 MMFSK transmitter using EOD.

4 122 JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 2, MARCH 2009 Fig.5 Modulation process. Fig.6 MMFSK Receiver using EOD. IV. OPERATION OF MAJORITY DECISION LOGIC There are m 2 group B hopping patterns in the receiver, and, every hopping pattern is subtracted individually from the second stage input signal. When subtract BBx B jb, the output is as shown in Fig.7. input becomes L h +(A i -A x )+B jb and even correctly subtract BBj, the output becomes L h +A i -A x as shown in Fig.8. Fig.8 Effect of first stage error. Fig.7 Output of wrong B. By checking all subtracted outputs through a majority decision logic we can find BBj and L h simultaneously. In the above example no error is postulated in the decision of A i, however, when a wrong choice of A x occurs in the first demodulation stage, second demodulation stage In such a case it is almost impossible to find correct L h and BBj, that is, when we specify EO patterns only for group A hopping patterns, errors arisen in the first demodulation stage influence the second demodulation stage. To prevent error propagation from the first demodulation stage to the second demodulation stage, binary hopping patterns are proposed in the following.

5 JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 2, MARCH V. BINARY HOPPING PATTERN Group A hopping patterns are characterized by their EO patterns only. There can be many hopping patterns which exhibit the same EO pattern and this gives some confusion in the design of group A hopping patterns. In this paper a new category of group A hopping pattern named binary hopping pattern is proposed. When we use binary hopping patterns, error propagation from the first demodulation stage to the second demodulation stage can be prevented by the majority decision logic in the second demodulation stage. Binary hopping patterns are defined as follows. That each level must take a value out of either one of two neighboring values. This makes the components of A i -A k 0 or ± 1. Table 1 shows a most simple example in which E=0, O=1. (Type 1) Table 1 Hopping patterns of (0, 1) levels.(type 1) A 1 E O E E O A 2 O E O E O A 1 -A Fig.9 Binary hopping pattern Type 3. To show the effectiveness of binary hopping patterns an illustrative example is given in Fig.10 and Fig.11. Table 2 shows another simple example in which E=4, O=5. (Type2) Table 2 Hopping patterns of (4, 5) levels.(type 2) A 1 E O E E O A 2 O E O E O A 1 -A Table 3 shows the other sophisticated but general example in which levels of every chips are made by different neighboring values. (Type 3) Table 3 Hopping patterns of (2,3)(6,5)(0,1)(4,5)(4,3) levels.(type 3) A 1 E O E E O A 2 O E O E O A 1 -A In this example, for the first chip E=2, O=3 for the second chip E=6, O=5 for the third chip E=0, O=1 for the forth chip E=4, O=5 for the fifth chip E=4, O=3 are assumed. Type 3 is as shown in Fig.9. Fig. 11 B j subtracted output. Fig.10 shows a transmitting signal in which A 1 in Table 3 is used as A i. When a wrong choice of A x occurs in the receiver, A x is also a binary hopping pattern as shown in Fig.11, the result is as shown in the figure. Fig.11 shows the BBj subtracted output, the other B xb subtracted output may exhibit a dispersed figure and it may be easy to find BBj through the majority logic. However, some ambiguity will remain for L h as B jb subtracted output will disperse L h ± 1. Fig. 10 Modulation by binary hopping pattern. (Type 3)

6 124 JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 2, MARCH 2009 VI. SOME PROBLEMS IN THE DESIGN OF GROUP A (1) Minimum Hamming Distance To make group A hopping pattern itself robust to noise, it is desirable to design EO patterns to have minimum Hamming Distance between codes more than three. When minimum Hamming Distance between codes is set three, effect of one character error can be removed perfectly. Fig.12 shows an example of error probability improvement by Hamming Distances. VII.SIMULATION RESULT To show the effectiveness of the binary hopping pattern a computer simulation was made. Specification of the simulation is as shown in Table 4. Hopping patterns used in the simulation are shown in Table 5. In Table5, hopping patterns with the same EO pattern are arranged on the same number row. Fig.13 shows a result of simulation. Table 4 Simulation model. Number of frequency level n 16 Number of chips L 7 Number of patterns(group A) 16 Number of patterns(group B) 64 Environment Detection AWGN coherent Level L h are even number levels only Fig. 12 Comparison of the error performances for minimum distance 3 and 1. (2) Inversion problem by adding odd level When an odd level is added to a group A hopping pattern, its EO pattern is reversed. For example a pattern EEEOE will be changed to its reciprocal pattern OOOEO. So that we must discard reciprocal patterns when we use both even levels and odd levels. Instead, if we use even levels only we can use all combinations of EO patterns, and further, ambiguity L h ±1 can be avoided and slight improvement in error probability can be expected. Fig. 13 Comparison of error performances for binary and non-binary. Table 5 Hopping patterns used.

7 JOURNAL OF COMMUNICATIONS, VOL. 4, NO. 2, MARCH VIII.CONCLUSIONS In this paper as a novel hopping pattern to improve the error probability characteristics of MMFSK-EOD system binary hopping patterns are proposed. By using binary hopping patterns, error propagation from the first demodulation stage to the second demodulation stage can be prevented and the system error performance is improved considerably. Principle of the error performance improvement is explained, and, to show the effectiveness of the binary hopping patterns, a result of computer simulation is given. Further error performance improving methods in the design of group A hopping patterns are also shown. REFERENCES [1] D. J. Goodman, P. S. Henry, V. K. Prabhu, "Frequency Hopping Multilevel FSK for Mobile Radio," Bell System Tech. J., vol.59, no.7, pp , Sept [2] G. Marubayashi, A Novel High Capacity Frequency Hopping Spread Spectrum System Suited to Power-Line Communications, Proceedings of ISPLC99, pp , April [3] S. Komatsu, M. Hamamura, and G. Marubayashi, Improved M-ary multilevel FSK power line transmission system, Proceedings of ISPLC2004, pp , April [4] G. Marubayashi, M. Hamamura, Extended Even-Odd Discrimination Method of Hopping Pattern Synthesis for MMFSK Power-Line Transmission System, Proceedings of ISPLC2005, vol1, pp , April [5] Y. Nojiri, S. Tachikawa, G. Marubayashi, Error Rate Simulation for a MMFSK System Using Even-Odd Discrimination Method, Proceedings of ISPLC2006, pp , March [6] Y. Nojiri, G. Marubayashi, S. Tachikawa, Spectral Efficiency and BER Performances of MMFSK System Using Even-Odd Discrimination Method, IEICE,WBS , pp.13-18, Oct.2006 [7] Y. Nojiri, S. Tachikawa, G. Marubayashi, Studies on Fundamental Properties of MMFSK Systems, Proceedings of ISPLC2007 pp , March 2007 [8] S.Hasegawa, S.Tachikawa, G.Marubayashi, MMFSK- EOD System using Binary Hopping Patterns, IEEE 12th International Symposium on Power Line Communications and its Applications (ISPLC2008),02-04 Apr 2008 Jeju Island Korea,37020,pp.234~238 Suguru Hasegawa was born in Fukushima, Japan on July 7, He received the B.S. degree in electrical engineering from Nagaoka University of Technology, Nagaoka, Japan, in He is now in the master course of electrical engineering, Nagaoka University of Technology. His current research interests are in spread spectrum communication systems and power line communication systems. Shin'ichi Tachikawa was born in Niigata, Japan, January, He received the B.S., M.S. and Dr. degrees in electrical engineering from Nagaoka University of Technology, Nagaoka, Japan, in 1980, 1982 and 1991, respectively. Since 1982, he has been a faculty member of Engineering at Nagaoka University of Technology, where he is now an Associate Professor. His current research interests lie in the areas of spread spectrum communication system, wide band system, coding theory and signal processing. Dr. Tachikawa is a member of IEEE, IEICE and SITA of Japan. Gen Marubayashi was born in shanghai china, sept He received a B.S. and a ph.d. from Kyoto University, Kyoto, Japan, in 1953 and 1968, respectively. From 1953 to 1978, he was with the Electrical Communication Laboratories, N.T.T., Japan, where he was engaged in the research and development of coaxial cable transmission and optical fiber transmissions. He joined Nagaoka University of Technology, Nagaoka, Japan, from 1978 to 1994 as a Professor of Electrical Engineering. His current interests are centered around the spread spectrum communication systems. He is a fellow of IEICE and of the Society of Information Theory and Its Applications, Japan. He received the OUTSTANDING SERVICE AWARD from IEEE, Apr.2008, for his power line communication research activities.